1. Technical Field of the Invention
The present invention relates generally to the field of integrated circuits and, more particularly, to a system and apparatus of temperature compensated slew rate control.
2. Description of Related Art
In interface integrated circuits, it is often desirable to control the transition speed, or slew rate, of digital outputs. Greater slew rates, and the corresponding shorter transition times, cause problems due to radiated noise, substrate injection, and crosstalk effects. In many applications, it is desirable to limit the rate at which outputs change states, especially when these outputs are rail-to-rail. Unfortunately, although there is often a need to limit the maximum slew rate, there also exists a minimum slew rate which may not be violated to insure the maximum data rate. In transceiver circuits, these limitations are typically applied to the receiver outputs, since these outputs are rail-to-rail, and must switch in a specified period to maintain the maximum guaranteed switching rate of the receiver.
Generally, current slew rate control circuits provide poor slew rate tracking over a varied temperature range. The control of the slew rate dramatically deteriorates when the associated integrated circuit is operated in a low temperaturexe2x80x94high supply voltage condition or fast-condition. In the fast-condition state, the output buffer current capability increases and the logic gate propagation delay decreases. In a high temperaturexe2x80x94low supply voltage or slow-condition, the associated increase in logic gate propagation delay causes an unacceptable increase in the switching times of the output buffers.
As with most design problems in semiconductor design, there are many ways to attempt to control slew rate for a rail-to-rail output. The chief objectives of slew rate control are to control the transition time of an output between a minimum and maximum rise/fall time specification. Generally it is desirable that the rise and fall times be evenly matched, that the output signal be reasonably free of distortion, and that the rise and fall time conditions are met over reasonably allowed conditions of temperature.
The present invention achieves technical advantages as an apparatus and system of temperature compensated slew rate control for interface type circuits. A switching device is controlled by a predrive which supplies a control signal that limits an output slew rate of the switching device and also varies proportionally to the operating temperature of the switching device.